Today seemed a good opportunity to look at some of the factors involved in writing large-group interactions. Here are some of the possible permutations (I don't claim to be comprehensive!).
1. The chaos crowd
In the chaos crowd, nobody is really interacting with anybody else. The chaos crowd is faceless, usually moving. I tend to approach a POV protagonist in a chaos crowd by establishing the chaos of the scene (magnitude, degree of movement) and thereafter relying on just the character's impressions of things that happen nearby. The character is as likely to encounter body parts and interact with them as to encounter whole people. Dialogue is usually either uttered to one companion or shouted at anybody within reach, so not a lot of complex management there.
2. The speechmaking situation
Here you have one person who is communicating something to the whole group. That person is the center of the interaction, as it were. If that person is the POV character, then you can have the speechmaker remark internally on reactions within the crowd of the audience. They may recognize some people in the audience and note those particular reactions. If the speechmaker is not the POV character, then there will be little sense of the reactions of individuals in the crowd, unless they are right next to the POV listener. However, the POV listener will definitely react personally to what the speechmaker is saying, and will also have an impression of the general mood of the crowd. Hecklers will be noticeable auditorily to either a POV speaker or a POV listener, but unlikely to be visually identifiable unless special conditions are in place.
3. The single-center group
This is a smaller group, with a single person who is the center of attention and/or perceived as the leader. The leader is more likely to be able to manage the conversation of the others, because he or she will be the one to whom most speech is directed. Every time you have active interaction between three or more characters where their identity is important, it's critical to keep track of the interactive style, favorite topics, and personality of each participant. Dialogue tags can be critical for keeping track of whom you're hearing at any given time.
4. The multi-center group
This is a group where all or most members are known, but where two (or more) people have come in with agendas and want to be the group center. Essentially what will happen is that these people will come in and make a move for the attention of the whole group, and then the group will split one way into smaller groups. In the scene I'm writing today, Garr grabs the attention of one person and Selemei gets the attention of the other two. What can happen here, however, is that other characters can make conversational or physical moves that either unify the entire group momentarily, or cause it to break up again into another configuration. There are a lot of possible permutations here, but they will be directly influenced by the agendas of the individuals involved. Some people will tend to stick to each other while others will not. I will remark, though, that groups of four or more will have an increasing tendency to break up into smaller subgroups.
The physical position of the people involved will also have a huge influence. If it's a cocktail party or other free-form situation, people's ability to form and reform smaller groups will be enhanced. If the entire party is seated around a large table, then one will tend to choose one of the people sitting nearest and begin a conversation. Speechmaking can be started from the head of the table, and larger groups can form between people who are around a table corner, or seated across from one another, etc.
The main point of this is to say that I don't often see large-group interactions taken advantage of in all their possible permutations. It's worth putting some thought into how this stuff can work, particularly if a lot of important characters are interacting at the same time.
I also suggest you look at Deborah J. Ross and Dave Trowbridge's comments below, for some great crowd-related questions and a book recommendation on the topic.